Industrial radiography is a non-destructive technique for testing and analyzing defects in components such as glass, paper, wood or metal parts. This technique is widely used in aeronautics, the nuclear industry or the petroleum industry since it makes it possible to detect welding defects or defects in the texture of materials in aircraft components, nuclear reactors or pipelines.
This technique consists of exposing a component to be analyzed to an ionizing radiation, in general X or gamma radiations having an energy between 10,000 and 15,000 kV, either directly or by means of an intensifying screen. It is therefore necessary with this technique to use specific radiographic elements which are sensitive to this ionizing radiation.
The sensitivity of the radiographic emulsions to X or gamma radiations is due to the absorption of a part of these radiations by the silver halide grains, which causes a secondary emission of electrons, which form an internal latent image. Consequently the ionization radiations have an action on the silver halide grains solely when they are absorbed by these grains.
However, it is known that the major part of the ionizing radiation passes through the silver halide grains without being absorbed. Only a very small part of the incident radiation (less than 1%) is absorbed and contributes to the formation of developable latent image nuclei.
It is for this reason that the elements for industrial radiography generally consist of silver halide emulsion comprising mainly thick grains (three dimensional or cubic) in order to be able to absorb the maximum amount of ionizing radiations passing through the emulsion layer.
In addition, in order to assist the absorption of the ionizing radiations, it is known that the silver content or the thickness of the emulsion layers can be increased, or that the radiographic support can be covered on each of its faces with a silver halide emulsion layer.
For example, the patent FR 2 367 300 describes a radiographic emulsion comprising silver halide grains having mean diameter of at least 0.25 .mu.m and a substantially regular crystalline structure. In the examples illustrating this patent, the silver halide emulsions consist of regular cubo-octahedral grains of at least 0.7 1 .mu.m.
For several years, there have appeared silver halide photographic elements consisting of tabular grains which offer sensitometric advantages such as, for example, an improved sensitivity/granularity ratio.
In more recent patents, attempts have therefore been made to introduce these tabular silver halide grains into industrial radiography elements.
For example, U.S. Pat. No. 5,230,993 describes a product for medical or industrial radiography which may contain tabular silver halide grains. However, as the examples show, this patent describes spectrally sensitized radiographic elements which are intended to be used with fluorescent intensifying screens which re-emit visible light when they are exposed to X-rays. In this case, the silver halide emulsions are conventional emulsions sensitive to visible light.
U.S. Pat. No. 4,883,748 describes a film for industrial radiography in which the silver halide emulsion comprises silver halide grains having an aspect ratio (the ratio between diameter and thickness) less than or equal to 5 (preferably between 1 and 3) and whose surface region contains a larger proportion of iodide than the internal region. In the examples illustrating the invention, the majority of the emulsions consist of tetradecahedral grains with an aspect ratio of 1. Example 2 shows clearly that by increasing the aspect ratio the contrast of the radiographic element exposed to X-rays is degraded.